Combustion Phenomena of High Temperature Air Combustion - High Temperature Air Combustion: From Energy Conservation to Pollution Reduction - page 75

burning velocity. This product is equivalent to the reaction rate integrated through
the reaction zone. According to the scheme employed here, all methane is converted
first into CH 3 in the course of the oxidation process. Then the step CH 4 → CH 3 is
chosen because its rate gives the overall rate of the methane combustion in the
investigated flames. In the Miller and Bowman scheme, the elementary reactions
contributing to this step are

CH 4 + M → CH 3 + H + M (R23)

CH 4 + O 2 → CH 3 + HO 2 (R24)

CH 4 + H → CH 3 + H 2 (R25)

CH 4 + OH → CH 3 + H 2 O (R26)

CH 4 + O → CH 3 + OH (R27)

From these, R23 and R24 progress with small rates so that the other three define
the total CH 4 consumption rate. The rate of each of the main reactions contributing
to the CH 4 → CH 3 step and the total CH 4 consumption rate for α = 0.06 and 0.15
are plotted in Figure 2.49 . The peak consumption rate of CH 4 at α = 0.06 is 0.006
mol/cm 3 ·s, about 1/3 of the value at α = 0.15. This is due to the dilution effect,
despite a temperature at the peak consumption rate in the α = 0.06 condition that
is 340 K higher than that in the α = 0.15 flame. Because the fuel flux at α = 0.06
is about twice that at α = 0.15, the reaction zone of the former flame has to be much
thicker than that of the latter flame. For the purpose of comparison, the reaction
zone thickness in flame under different conditions is defined here as the ratio between
the difference T a - T 0 and the maximum value of dT / dx . The variation of fuel flux
and reaction zone thickness with α is shown in Figure 2.50 ; both are seen to change
steeply for α < 0.09.
The same was done for T a = 2000 K flames, and the results are shown in Figure
2.51 . The fuel flux shows a maximum, and the values are much smaller than those
of Figure 2.50 . This can be ascribed to the fact that no flames capable of self-ignition
are obtained for T a = 2000 K, so that preheating and diluting cannot increase the
fuel flux.

2.3.2.2.4 NO Formation
The NO concentration at 10 mm after the flame front was found to be almost constant
with α for T a ≤ 2000 K, but for T a = 2200 and 2400 K, the NO level decreased as
α decreased.
The rates of the main reactions producing NO and their sum are plotted in Figure
2.52 . I n contrast to the reactions belonging to the step CH 4 → CH 3 , the NO producing
reactions are not inhibited by dilution of the mixture. This can be ascribed to the
fact that as α decreases, the amount of N 2 in the mixture does not change and also
the initial temperature increases. Many reactions related to NO formation comprise